chore: update toolset and promptset initialize failure log (#2039)

## Description

Update error log to more details when initialization fails.
* When a toolset name is not valid, show the toolset name (similar with
promptset).
* When tool does not exist, show the tool name that is missing (similar
with prompt).

## PR Checklist

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- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [x] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)
- [x] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #1579

internal/prompts/promptsets.go

@@ -46,7 +46,7 @@ func (t PromptsetConfig) Initialize(serverVersion string, promptsMap map[string]
 	var promptset Promptset
 	promptset.Name = t.Name
 	if !tools.IsValidName(promptset.Name) {
-		return promptset, fmt.Errorf("invalid promptset name: %s", t)
+		return promptset, fmt.Errorf("invalid promptset name: %s", promptset.Name)
 	}
 	promptset.Prompts = make([]*Prompt, 0, len(t.PromptNames))
 	promptset.McpManifest = make([]McpManifest, 0, len(t.PromptNames))
@@ -57,7 +57,7 @@ func (t PromptsetConfig) Initialize(serverVersion string, promptsMap map[string]
 	for _, promptName := range t.PromptNames {
 		prompt, ok := promptsMap[promptName]
 		if !ok {
-			return promptset, fmt.Errorf("prompt does not exist: %s", t)
+			return promptset, fmt.Errorf("prompt does not exist: %s", promptName)
 		}
 		promptset.Prompts = append(promptset.Prompts, &prompt)
 		promptset.Manifest.PromptsManifest[promptName] = prompt.Manifest()

internal/tools/toolsets.go

@@ -46,9 +46,9 @@ func (t ToolsetConfig) Initialize(serverVersion string, toolsMap map[string]Tool
 	var toolset Toolset
 	toolset.Name = t.Name
 	if !IsValidName(toolset.Name) {
-		return toolset, fmt.Errorf("invalid toolset name: %s", t)
+		return toolset, fmt.Errorf("invalid toolset name: %s", toolset.Name)
 	}
-	toolset.Tools = make([]*Tool, len(t.ToolNames))
+	toolset.Tools = make([]*Tool, 0, len(t.ToolNames))
 	toolset.Manifest = ToolsetManifest{
 		ServerVersion: serverVersion,
 		ToolsManifest: make(map[string]Manifest),
@@ -56,7 +56,7 @@ func (t ToolsetConfig) Initialize(serverVersion string, toolsMap map[string]Tool
 	for _, toolName := range t.ToolNames {
 		tool, ok := toolsMap[toolName]
 		if !ok {
-			return toolset, fmt.Errorf("tool does not exist: %s", t)
+			return toolset, fmt.Errorf("tool does not exist: %s", toolName)
 		}
 		toolset.Tools = append(toolset.Tools, &tool)
 		toolset.Manifest.ToolsManifest[toolName] = tool.Manifest()